Nuclear reactors such as pressurized water reactors comprise a core consisting of fuel assemblies placed adjacent to each other within the reactor vessel. A fuel assembly comprises a bundle of fuel rods held in a supporting structure called a skeleton assembly which comprises the frame for the assembly. This skeleton assembly in particular includes guide tubes located in the axial direction of the fuel assembly connecting the top and bottom ends and supporting the spacer grids for the fuel rods. The purpose of these guide tubes is to ensure that the framework has satisfactory rigidity and to allow the assembly of neutron-absorbing rods used to control the reactivity of the nuclear reactor core to be inserted.
The absorber rods are connected together at their top ends by a support which is generally called a “spider”, to form a bundle called a control cluster. The set of absorber rods can move within the guide tubes of the fuel assembly.
In order to regulate the reactivity of the nuclear reactor core while the reactor is in operation the vertical positions of the control clusters within particular assemblies of the core are changed, either so that they are inserted, the control cluster being then moved downwards, or extracted, the control cluster being then moved upwards, so that a variable length of absorber rod is inserted into the core assemblies. Control clusters of different types are generally used in different parts of the nuclear reactor core to control the core's reactivity and the power distribution within the reactor core while the nuclear reactor is in operation. Highly absorbent clusters, black clusters, and less absorbent clusters, grey clusters, are used in particular.
In general the absorber rods comprise a tube closed at its upper end by a first end plug called a top end plug and at its bottom end by a second plug called a bottom end plug for the rod. The absorber rods are secured to the holding spider through their top end plugs.
Generally, in the case of black clusters the rod assembly comprises rods having a high neutron absorption capacity. These absorber rods may comprise a cladding tube enclosing pellets of an absorbent material such as boron carbide B4C, tubes of a neutron-absorbing material which do not enclose absorbent pellets, or again tubes of absorbent material enclosing pellets of boron carbide B4C. In particular it has been suggested that hafnium tubes should be used as tubes of absorbent material for the rods in control clusters. Clusters adjusting the reactivity of nuclear reactors may therefore consist wholly or in part of absorber rods comprising a hafnium tube which may include pellets of an absorbent material such as B4C. In some circumstances it has been suggested that only a part of the absorber rods, for example the bottom part, should be made of hafnium.
Grey clusters include both absorber rods and inert rods consisting of a simple tube of a material which is not absorbent or has little absorbency, closed by end plugs at its extremities. Absorber rods may comprise tubes of absorbent material such as hafnium.
Hafnium has the advantage over other absorbent materials that it has excellent compatibility with the primary fluid, shows little swelling under irradiation and has good creep resistance at the operating temperature of a pressurized water nuclear reactor. It can therefore be used without any sheathing.
However, hafnium can only be welded to alloys of the same family (titanium, zirconium, hafnium) or alloys forming continuous solid solutions with hafnium.
If hafnium is used for the top end plug, the mechanical strength of the control cluster is not optimal because hafnium does not have sufficiently good mechanical properties for the stresses experienced by the cluster when in operation. Furthermore, the use of a hafnium plug in the top part of an absorber rod is not really justified on the grounds of neutron absorption, given that the top plug is only exposed to a very low neutron flux because it remains above the top of the core. Finally, the use of hafnium for the top end plug is accompanied by an increase in the mass of the cluster, and this may constitute a strong operational constraint. The use of zirconium alloy for the top end plug would be compatible with the mass requirements without any deterioration in absorbency. However, the mechanical properties of these alloys are also inadequate. Conversely, the properties of titanium alloys are perfectly compatible with the performance required.
As far as the bottom end plug is concerned, the use of hafnium is not ruled out for mechanical strength reasons because the properties of this material are compatible with the mechanical stresses applied to that component. In this area where there is a high neutron flux it is useful to have neutron absorption capacity. Finally as the volume of the bottom end plug is small, the resulting increase in mass is small and compatible with the requirements for the mass of control clusters. The bottom end plug may therefore be made of hafnium, or a zirconium alloy, while remaining compatible with functional requirements.